The present invention relates generally to three-phase power line communication systems and, more particularly, to an apparatus that is coupled to a power line system to detect both voltage and current signals.
Power line communication systems utilize transmitters and receivers to communicate between remote stations that are connected in signal communication with the power line. Transmission of a message requires some form of modulation and a means for injecting the modulated signal onto the power line. Various types of modulation can be used in conjunction with a power line communication system. For example, phase shift keyed (PSK) modulation involves the use of a carrier signal with a constant frequency, such as 12.5 kilohertz, and a base band data message that is a binary representation of information. To provide a phase shift keyed modulated signal containing the base band data message, the carrier signal and the base band data signal are connected, as inputs, to an exclusive-OR device. The output of this exclusive-OR device, or modulator, is then amplified and injected onto the power line. Reception of a phase shift keyed modulated message from a power line communication system utilizes receiver circuitry that is capable of removing lower frequencies from the signal and then shaping the signal in such a way so as to be appropriate for demodulation. Typically, the incoming signal is passed through a high pass filter to remove the lower frequencies, such as the power transmission frequency of .alpha.Hertz, and then it is hard limited in order to provide a generally square wave signal for input to a demodulator.
U.S. Pat. No. 3,911,415, which issued on Oct. 7, 1975 to Whyte, discloses a distribution network power line carrier communication system that can be used for linking individual power customers with a central station. Frequency translating and signal reconditioning repeaters are connected to intermediate locations of the network to relay carrier signals at different frequencies. U.S. Pat. No. 3,942,168, which issued on Mar. 2, 1976 to Whyte, discloses a distribution network power line communication system that includes a central communication terminal at a distribution substation. The central communication terminal is connected in signal communication with a plurality of remote communication terminals on the electrical distribution power lines. U.S. Pat. No. 4,357,598, which issued on Nov. 2, 1982 to Melvin, discloses a three-phase power distribution network communication system that comprises a plurality of remote devices, with certain of the remote devices being used as signal repeaters. The signal repeaters are each coupled to at least two of the three-phase conductors and each signal repeater includes circuitry for producing a composite signal in response to the coupled signals. U.S. Pat. No. 3,967,264, which issued on June 29, 1976 to Whyte et al., discloses a distribution network power line communication system that is divided into addressable communication zones defined by repeaters located at the distribution transformers of the distribution network. Each repeater is uniquely addressable by an interrogation signal. U.S. Pat. No. 4,427,968, which issued on Jan. 24, 1984 to York, discloses a distribution network communication system with flexible message routes. A plurality of signal repeaters are each connected with certain of the remote terminals through a distribution network and each signal repeater contains stored route and role codes as well as a unique address code allowing each signal repeater to be additionally addressed as an end device. U.S. Pat. Nos. 3,911,415; 3,942,168; 4,357,598; 3,967,264 and 4,427,968 are hereby incorporated by reference.
Various methods for injecting the modulated signal onto the power line are known to those skilled in the art. U.S. Pat. No. 4,323,882, which issued on Apr. 6, 1982 to Gajjar, discloses an apparatus for inserting carrier frequency signal information onto distribution transformer primary windings. Similarly, various types of receiver configurations for receiving signals from a power line communication system are known to those skilled in the art. U.S. Pat. No. 4,355,303, which issued on Oct. 19, 1982 to Phillips et al., discloses a receiver for use with a distribution network power line carrier communication system that is magnetically coupled to a distribution power line. It comprises a receiver amplification circuit that includes an automatic gain control circuit to prevent saturation of the receiver electronics along with a feedback circuit to determine the gain. U.S. Pat. No. 4,382,248, which issued on May 3, 1983 to Pai, discloses a remote device for a multi-phase power distribution network communication system. It includes a circuit for independently receiving each of the communication signals carried by the phase conductors of the power line system. The receiving circuit produces an input signal having a serial format in response to each of the received signals.
After signals are received from a power line communication system, the signals must be demodulated to interpret the messages contained therein. Depending on the particular type of modulation used, the modulators will vary significantly. If a phase shift keyed modulation system is utilized, various types of demodulators can be employed. U.S. Pat. No. 4,311,964, which issued on Jan. 19, 1982 to Boykin, discloses a coherent phase shift keyed demodulator for power line communication systems that comprises means for sequentially processing plus and minus polarity samples of plural carrier segments occurring within each carrier data symbol. These samples provide a binary coded signal for producing corresponding first and second relative phase angle vector signals which are summed over several data symbols to generate reference phase angle signal vector signals. U S. Pat. No. 4,379,284, which issued on Apr. 5, 1983 to Boykin, discloses an improved demodulator that is applicable to systems using phase shift keyed demodulation methods. U.S. Pat. Nos. 4,355,303; 4,382,248; 4,311,964 and 4,379,284 are hereby incorporated by reference.
Distribution power line carrier communication systems must be designed in such a way that they are able to communicate between remote devices under many different conditions and system configurations. The remote devices must be able to communicate properly in spite of many different types of propagation degradations that can occur within a power line network. Classical transmission line problems of impedance mismatch and standing waves are common within the complex physical layouts of distribution wiring systems. In a typical system, feeder circuits include various unterminated lengths which can cause conventional signal detection methods to be inadequate. Furthermore, besides the transmission attenuation and propagation problems, the distribution feeder system offers a background noise characteristic which is often difficult to predict. Typically, the worst noise problems on a system result from one or two sources of high power electronic industrial controls and this type of electrical noise tends to attenuate as the distance from the source is increased.
In power line transmission systems where classical standing wave characteristics are present, the voltage and current are out of phase. In typical receiver apparatus, couplers are used to detect voltage signals on at least two of the three phase conductors. U.S. Pat. No. 4,382,248 illustrates this method. U.S. Pat. No. 4,573,170, which issued on Feb. 25, 1986 to Melvin et al, discloses a time diversity carrier signal sampler that utilizes a shift register in conjunction with a timer and a plurality of phase samplers. It measures the instantaneous logic level of each phase signal on a time diversity basis in order to avoid the disadvantageous effects of noise pulses which can occur coincidentally on all three phases. The voltage signals are separately received from the three-phase conductors and processed by a sampling circuit which treats each of the signals individually to determine the best phase source for receiving the incoming message. If this type of apparatus is coupled to the power line at a point where a standing wave condition exists, the strength of the voltage signal will be dependent upon the precise location along the standing wave at which the receiver is coupled to the three-phase system. If the receiver is coupled at an antinode of the voltage signal, a strong signal will be available for reception. However, if the receiver system is connected to the power line at a voltage node, a very weak signal will be received and the proper operation of the receiver and demodulator will be severely jeopardized.
The present invention takes advantage of the fact that, in classical standing wave situations, the voltage and current signals are out of phase. If a receiver is coupled to the power line at a point where a voltage signal node occurs, a current signal antinode will exist at the same location. If a means is provided for receiving both current signals and voltage signals from the transmission line, the problems incumbent with standing wave situations can be avoided. Furthermore, if the voltage and current signals are compared to determine the strongest signal, the receiver could be equally effective regardless of the particular location along the standing wave it is located. The present invention provides a means for detecting both voltage and current signals and for providing those signals to a device capable of comparing the signals and selecting the stronger of the two.
The present invention relates generally to the need to sense both voltage and current signals at any particular physical location along transmission lines in order to combat classical standing wave problems. The present invention utilizes voltage coupling to each of the phase conductors of a three phase power line system along with current coupling to the neutral of the power line. It takes advantage of the fact that, in standing wave patterns, either the voltage or current can be depressed, but standing waves do not cause both signals to be depressed at the same physical location along the transmission line.
Each of the three phase conductors is coupled to a common point and that common point is connected in signal communication with an input to a receiver. The common connection of the three phase signals to a common point has the affect of creating a composite voltage signal that represents a combination of all three signals. A signal coupling unit is connected between the common connection point and the receiver input. The function of the signal coupling unit is to permit the 60 Hertz power line frequency to pass to ground while the frequency modulated signals pass to the receiver input. This portion of the present invention provides a single composite voltage signal received from the three phase conductors of the power line system.
An H-field coupler is operatively associated with the neutral conductor of the power line system. It utilizes a ferrite core with a plurality of turns around it. The core assembly is placed proximate the neutral conductor and provides a current signal to a second input of the receiver. The receiver therefore has two signal inputs to compare and select the strongest signal. If the apparatus is coupled to the power line at a location where the voltage signal is at a node, or null point, the current signal will be at an antinode point if a classical standing wave situation exists. Therefore, the receiver and related circuitry has two signals available to it. If the voltage signal is weak, the current signal will be strong and vice versa. Regardless of the specific location where the receiver circuitry is coupled to the transmission line, one of the two signals will be chosen based on their relative strengths and the likelihood of successful reception is enhanced.
In applications where the receiver circuitry must also be accompanied with signal transmission capability, the present invention utilizes the common point during transmissions. The output of a transmitter is connected in signal communication with the common point and modulated signals, which have been amplified, are injected onto the power line through the phase couplers connected between the phase conductors and the common point. Therefore, in situations where transceivers are required, the present invention provides both receiving and transmitting means.